Neisseria meningitidis is a causative agent of bacterial meningitis and sepsis. Meningococci are divided into serological groups based on the immunological characteristics of capsular and cell wall antigens. Currently recognized serogroups include A, B, C, W-135, X, Y, Z and 29E. The polysaccharides responsible for the serogroup specificity have been purified from several of these groups, including A, B, C, W-135 and Y.
N. meningitidis serogroup B (termed “MenB” or “NmB” herein) accounts for a large percentage of bacterial meningitis in infants and children residing in the U.S. and Europe. The organism also causes fatal sepsis in young adults. In adolescents, experimental MenB vaccines consisting of outer membrane protein (OMP) vesicles are somewhat protective. However, no protection has been observed in vaccinated infants, the age group at greatest risk of disease. Additionally, OMP vaccines are serotype- and subtype-specific, and the dominant MenB strains are subject to both geographic and temporal variation, limiting the usefulness of such vaccines.
Effective capsular polysaccharide-based vaccines have been developed against meningococcal disease caused by serogroups A, C, Y and W135. However, similar attempts to develop a MenB polysaccharide vaccine have failed due to the poor immunogenicity of the capsular MenB polysaccharide (termed “MenB PS” herein). MenB PS is a homopolymer of (N-acetyl (α 2->8) neuraminic acid. Escherichia coli K1 has the identical capsular polysaccharide. Antibodies elicited by MenB PS cross-react with host polysialic acid (PSA). PSA is abundantly expressed in fetal and newborn tissue, especially on neural cell adhesion molecules (“NCAMs”) found in brain tissue. PSA is also found to a lesser extent in adult tissues including in kidney, heart and the olfactory nerve. Thus, most anti-MenB PS antibodies are also autoantibodies. Such antibodies therefore have the potential to adversely affect fetal development, or to lead to autoimmune disease.
MenB PS derivatives have been prepared in an attempt to circumvent the poor immunogenicity of MenB PS. For example, C3-C8 N-acyl-substituted MenB PS derivatives have been described. See, EP Publication No. 504,202 B, to Jennings et al. Similarly, U.S. Pat. No. 4,727,136 to Jennings et al. describes an N-propionylated MenB PS molecule, termed “NPr-MenB PS” herein. Mice immunized with NPr-MenB PS glycoconjugates were reported to elicit high titers of IgG antibodies. Jennings et al. (1986) J. Immunol. 137:1708. In rabbits, two distinct populations of antibodies, purportedly associated with two different epitopes, one shared by native MenB PS and one unshared, were produced using the derivative. Bactericidal activity was found in the antibody population that did not cross react with MenB PS. Jennings et al. (1987) J. Exp. Med. 165:1207. The identity of the bacterial surface epitope(s) reacting with the protective antibodies elicited by this conjugate remains unknown. Also, because a subset of antibodies elicited by this vaccine have autoreactivity with host polysialic acid (Granoff et al. (1998) J. Immunol. 160:5028) the safety of this vaccine in humans remains uncertain.
Despite these attempts, conventional approaches have failed to identify antigens that are safe and capable of conferring broad protection against MenB infection.
There has been considerable interest in using molecular mimetic antigens to elicit protective immune responses to various pathogens, as well as for the treatment of cancer and autoimmune diseases. This approach to vaccine development for the prevention of infectious diseases has the greatest utility when the nominal antigen is toxic or difficult to purify, or when it is desirable to direct the immune response to a limited number of epitopes. Nevertheless, there are relatively few studies that report success of a mimetic vaccine in eliciting protective antibodies to a pathogen.
A number of functionally active antibodies directed against MenB PS derivatives have been described in U.S. Pat. No. 6,048,527. These antibodies do not cross-react, or are minimally cross-reactive with host tissues, and thus pose minimal risk of evoking autoimmune disease. U.S. Pat. No. 6,030,619 describes molecular mimetics of unique epitopes of MenB PS identified using these antibodies. However, the discovery of peptide mimetics of other MenB antigens remains of considerable interest.
The complete genomic sequence of MenB, strain MC58, has been described. Tettelin et al., Science (2000) 287:1809. Several proteins that elicited serum bactericidal antibody responses have been identified by whole genome sequencing. These proteins have conserved sequences and appear to be surface-exposed on encapsulated MenB strains. Pizza et al., Science (2000) 287:1816. One of these proteins is GNA33 (genome derived antigen). GNA33 is a lipoprotein and the predicted amino acid sequence shows homology with a membrane-bound lytic murein transglycosylase (MltA) from E. coli and Synechocystis sp. Lommatzsch et al., J. Bacteriol. (1997) 179:5465-5470. GNA33 is highly conserved among Neisseria meningitidis. Pizza et al., Science (2000) 287:1816. Mice immunized with recombinant GNA33 developed high serum bactericidal antibody titers measured against encapsulated MenB strain 2996. The magnitude of the antibody response was similar to that of control animals immunized with OMP vesicles prepared from strain 2996. However, the mechanism by which GNA33 elicits protective antibody was not identified, nor was the breadth of the protective response to different MenB strains.
It is readily apparent that the production of a safe and effective vaccine against MenB would be particularly desirable.